U.S. Pat. No. 3,208,860, the basic patent in the field of photochromic glass, provides a general discussion of the theoretical concepts underlying the phenomenon of photochromism in glass (formerly termed phototropism), as well as the practical considerations which must be observed in producing such glass. As illustrative of the phenomenon, the patent provides exemplary compositions of inorganic silicate glasses which contain submicroscopic crystals of at least one of the silver halides, viz., silver chloride, silver bromide, or silver iodide. When subjected to actinic radiation, customarily ultraviolet radiation, the crystals become darker in color, thereby decreasing the optical transmission of the glass. However, when the actinic radiation is removed, the crystals return to their original state and the optical transmission of the glass is restored.
It is postulated that a reaction occurs between the actinic radiation and the crystals dispersed within the glassy matrix which, in some manner, alters the absorptive characteristics of the crystals to visible light. Unlike the normal photographic film, glass is inert and impermeable. Therefore, when the impingement of actinic radiation on the glass is halted, the crystals are permitted to return to their original state, since the reaction products resulting from the exposure of the crystals to actinic radiation cannot diffuse away from the site of the reaction. This circumstance allows the transmission of such glass to visible light to be reversibly varied ad infinitum.
U.S. Pat. No. 3,325,299 discloses the production of photochromic glass from inorganic glass compositions comprising copper, cadmium, and a halogen selected from the group consisting of chlorine, bromine, and iodine as essential components. The photochromically-active constituent of these glasses is a copper-cadmium halide phase which darkens on exposure to 300-550 mm radiation and fades in the absence thereof.
Glasses of the type described in U.S. Pat. No. 3,325,299 offer certain advantages over photochromic glasses of the silver halide type, principally with respect to batch cost and photochromic behavior. The darkening of these glasses is more proportional to the intensity of incident light and less dependent upon temperature than is the case with silver halide glasses. Also, these glasses tend to darken only in a relatively thin surface layer of the exposed glass, thus exhibiting high darkened optical density even in thin cross-section.
Unfortunately, photochromic glasses of the copper-cadmium halide type also suffer certain disadvantages, especially with respect to the color and bleaching characteristics of the glass. Thus these glasses in general exhibit only greenish coloration in the darkened state, and are not usually bleachable to the clear state by exposure to long wave-length visible light. These factors demonstrate the clear distinctions which exist between the coloration mechanisms and the photochromic response characteristics of these two different types of photochromic glasses.
U.S. Pat. No. 3,328,182 discloses silver halide photochromic glasses wherein cadmium oxide is added to the composition to improve the fading characteristics of the glass. Although it was found that minor amounts of this additive were effective to obtain accelerated fading, larger additions appeared to retard the fading rate and therefore were specifically discouraged. Aside from an improved thermal fading rate, it does not appear that these glasses exhibited photochromic properties substantially different from other silver halide-containing photochromic glasses.